issue with accessing GET parameters net/http in golang

Question

Following is my go program to extract GET parameters. (URL: /mysql?hostname=example.com)

package main

import (
        "net/http"
        "fmt"
        //"encoding/json"
        //"html"
        "github.com/kr/pretty"
);

func main(){
        http.HandleFunc("/", foo)
        http.ListenAndServe(":80", nil)
}

func foo(w http.ResponseWriter, r * http.Request){
        w.Header().Set("Server","A Go WebServer")

        w.Header().Set("Content-Type", "text/html")

        hostname := r.URL.Query()["hostname"]
        //w.Write([]byte(hostname[0]))
        fmt.Printf("%# v", pretty.Formatter(hostname[0]))


        w.Write([]byte(hostname))
}

Error shown on console.

"example.com"2014/09/01 02:57:22 http: panic serving 172.17.92.14:49411: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228)
/root/go/src/api/main.go:34 (0x400d77)
    foo: fmt.Printf("%# v", pretty.Formatter(hostname[0]))
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb)
2014/09/01 02:57:22 http: panic serving 172.17.92.14:49412: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228)
/root/go/src/api/main.go:34 (0x400d77)
    foo: fmt.Printf("%# v", pretty.Formatter(hostname[0]))
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb)
2014/09/01 02:57:22 http: panic serving 172.17.92.14:49413: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42dc41)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f8b5)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410381)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x410228)
/root/go/src/api/main.go:34 (0x400d77)
    foo: fmt.Printf("%# v", pretty.Formatter(hostname[0]))
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421bed)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422a6f)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x421a00)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d9bb)

I am having trouble with determining the data type of hostname variable. I should get it as a string.

When I try to write to http response writer

w.Write([]byte(hostname))

following build error is shown

go/src/api/main.go:38: cannot convert hostname (type []string) to type []byte

If I treat it as a string array build is successful but there is run time error

        newhost := hostname[0];
        w.Write([]byte(newhost))

Error:

    2014/09/01 04:42:40 http: panic serving 172.17.92.14:50404: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e)
/root/go/src/api/main.go:38 (0x400d63)
    foo: newhost := hostname[0];
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1)
2014/09/01 04:42:40 http: panic serving 172.17.92.14:50405: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e)
/root/go/src/api/main.go:38 (0x400d63)
    foo: newhost := hostname[0];
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1)
2014/09/01 04:42:40 http: panic serving 172.17.92.14:50406: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42db27)
    _func_003: buf.Write(debug.Stack())
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:1443 (0x40f79b)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:128 (0x410267)
/home/michael/DPKG/golang/src/pkg/runtime/runtime.c:85 (0x41010e)
/root/go/src/api/main.go:38 (0x400d63)
    foo: newhost := hostname[0];
/usr/lib/go/src/pkg/net/http/server.go:690 (0x421ad3)
    HandlerFunc.ServeHTTP: f(w, r)
/usr/lib/go/src/pkg/net/http/server.go:926 (0x422955)
    (*ServeMux).ServeHTTP: mux.handler(r).ServeHTTP(w, r)
/usr/lib/go/src/pkg/net/http/server.go:656 (0x4218e6)
    (*conn).serve: handler.ServeHTTP(w, w.req)
/home/michael/DPKG/golang/src/pkg/runtime/proc.c:271 (0x40d8a1)

I also checked length of newhost as @VonC suggested and it gave expected results.

        fmt.Print("\n")
        fmt.Println((len(newhost)))

Output on console

"example.com"
11
2014/09/01 05:05:15 http: panic serving 172.17.92.14:50779: runtime error: index out of range
/usr/lib/go/src/pkg/net/http/server.go:576 (0x42dd33)

See also stackoverflow.com/a/29237567/32453 (go GET parameters example, also that question mentions POST parameters) — rogerdpack
– rogerdpack, Commented Mar 29, 2016 at 0:08

Tom Manterfield · Accepted Answer · 2017-01-25 11:22:37Z

53

You need to use url/Values.Get, it will return the empty string if the value isn't set:

hostname := r.URL.Query().Get("hostname")
if len(hostname) != 0 {
    io.WriteString(w, hostname)  // or
    w.Write([]byte(hostname))
}

// edit

Let's take it a step by step:

go/src/api/main.go:38: cannot convert hostname (type []string) to type []byte:

req.URL.Query["hostname"], returns a string slice ([]string) which you can't directly convert to []byte, only string can be converted like that.

The first runtime error: index out of range:

You're trying to use hostname[0] without checking how many elements in the slice, so in that case there weren't any elements.

That last runtime error: index out of range is weird though, I have no idea why you got it, must be something else you changed in the code.

edited Jan 25, 2017 at 11:22

Tom Manterfield

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answered Sep 1, 2014 at 13:31

OneOfOne

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2 Comments

Kasinath Kottukkal Over a year ago

That did the trick and worked like charm. I wish someone could explain what was the mistake.

OneOfOne Over a year ago

@KasinathKottukkal I added an explanation.

dyoo · Accepted Answer · 2014-09-01 16:30:43Z

7

If you only care about the first value, and you don't have to distinguish between POST or GET, you might consider using the simpler Request.FormValue function.

hostname := r.FormValue("hostname")

answered Sep 1, 2014 at 16:30

dyoo

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